The present invention relates to a workpiece holder for polishing, a polishing apparatus and a polishing method, which are used for precision polishing of a surface of workpiece such as a semiconductor wafer.
As a method for processing semiconductor wafers, in general, there are successively performed a slicing step to slice a thin disc (wafer) from a single crystal ingot, a chamfering step to prevent cracking of a peripheral edge portion of the wafer, a lapping step to eliminate unevenness of the wafer thickness, an etching step to remove a mechanical damage or contaminants, a polishing step to make the chamfered portion and the main surface of the wafer into mirror surfaces and so forth.
In the conventional polishing operation, a plate composed of a rigid material such as glass, metal and ceramics is used as a workpiece holder, and a workpiece is held by adhering it on the surface of the holder with an adhesive such as wax, or by vacuum adsorption using a workpiece holder surface composed of a gas-permeable porous material or a workpiece holder surface provided with multiple perforated holes.
However, if a workpiece is held directly on a workpiece holder surface composed of metal, ceramics or the like, stains may be generated on the surface of the workpiece, or scratches may be generated when the holder is harder than the workpiece. Therefore, in order to prevent such scratches and stains on the back surface of the workpiece and to obtain a workpiece having good flatness, the surface of the workpiece holder may be coated with a film of resin such as acrylic resin, polycarbonate resin and epoxy resin. Further, a method of the so-called co-rubbing polishing is performed, in which the resin coated on the surface for holding the workpiece is directly polished with a polishing pad before polishing of the workpiece so that the surface condition of the resin should match a surface condition of the polishing pad, and then the workpiece is held on the workpiece holder and polished.
An example of conventional workpiece holder for polishing is shown in FIGS. 5(a) and (b).
This workpiece holder 1 for polishing is constituted by a workpiece holder body 2 having a workpiece holding surface 15 and multiple perforated holes 4 for vacuum adsorption, and a workpiece holder back plate 5 disposed on the back side of the body. The perforated holes 4 are communicated to a vacuum apparatus not shown in the figure via a space 6 located between the workpiece holder body 2 and the workpiece holder back plate 5, and a vacuum way 7, and the workpiece is adsorbed and held on the workpiece holding surface 15 upon generation of vacuum. The workpiece holding surface 15 of the workpiece holder body 2 is coated with a resin coating film 3 that has perforated holes 4.
Upon polishing of a wafer, a workpiece is held on the workpiece holding surface 15 of the workpiece holder 1 for polishing by vacuum adsorption or the like, and the holder is mounted on the polishing head (not shown) having a rotation shaft. The holder is rotated by the polishing head and simultaneously presses the workpiece against a polishing pad adhered on a rotating polishing turn table at a predetermined load. A polishing agent is fed from a nozzle at a predetermined flow rate on the polishing pad, and then fed between the workpiece and the polishing pad, and thus the workpiece is polished.
When such polishing is performed, heat is generated during the polishing. If the polishing is performed by using such a workpiece holder having the configuration shown in FIG. 5, in particular, the heat generated during the polishing of the workpiece is gradually transferred to the workpiece, the workpiece holding surface (the portion of resin coating film) and the workpiece holder body in this order. And heat will be accumulated when the polishing is repeated.
However, since the workpiece holding surface is subjected to cleaning with a brush and so forth with supplying a cleaning solution in order to remove stains on the surface immediately before the polishing operation, the surface is cooled at this stage. On the other hand, in the workpiece holder body, in particular, as for the back side (the side of the back plate 5), heat will be gradually accumulated. For this reason, the amounts of the accumulated heat and radiated heat differ at the front surface and the back surface of the workpiece holder body. Therefore, when the number of the polishing operation (batch number of polishing operation) increases, there begins to be generated a temperature difference between the workpiece holding surface side and the back side of the workpiece holder body, and thus thermal deformation is caused in the whole workpiece holder body. Such deformation of the whole workpiece holder body causes a problem that it results in degradation of flatness of a workpiece held on it and polished.
In order to solve such a problem, a material showing low thermal expansion might be conventionally used for the workpiece holder body. However, under the current situation where a workpiece of extremely high degree of flatness is desired, only use of this means still leaves many insufficient points.
For the polishing, not only appropriate thermal expansion characteristics, but also rigidity enough to stand the polishing pressure is required. If these thermal expansion and rigidity as well as cost are taken into consideration, it is difficult under the present condition to obtain a workpiece holder for polishing that fulfills all of these requirements.
Further, if the surface of the workpiece holder is coated with a film of resin such as acrylic resin, polycarbonate resin and epoxy resin in order to prevent scratches and stains on the workpiece and to obtain a workpiece showing good flatness as described above, thermal deformation is likely to be significantly caused by the heat generation during the polishing, since the thermal characteristics of the resin (coefficient of thermal expansion and heat conductivity) are significantly differ from those of the material used for the workpiece holder body (for example, ceramics such as SiC). Thus, it is difficult to obtain high flatness.
Therefore, the present invention was accomplished in view of the aforementioned problems, and its main object is to provide a workpiece holder for polishing, a polishing apparatus and a polishing method, which can, in polishing utilizing a workpiece holder having such a configuration as described above, provide a workpiece having good flatness and showing a good magic mirror image by suppressing thermal deformation of the workpiece holder body and deformation of a resin film coated on the workpiece holding surface without degrading flatness of a workpiece held on the workpiece holder in the polishing of the workpiece, even when the number of polishing operation (batch number of polishing operation) increases.
In order to achieve the aforementioned object, the workpiece holder for polishing of the present invention is a workpiece holder for polishing comprising at least a workpiece holder body having multiple perforated holes for holding a workpiece by vacuum adsorption and a back plate disposed on the back side of the body, which is provided with temperature controlling means or cooling means for the holder body.
If such temperature controlling means or cooling means is used, the temperature difference between the workpiece holding surface and the back surface of the workpiece holder body and the temperature difference between a center portion and a peripheral portion of the workpiece holder body are eliminated, and thus the temperature becomes substantially uniform for the whole workpiece holder body. Therefore, thermal deformation of the workpiece holder body and the workpiece is hardly caused, and thus a workpiece showing high flatness can be obtained. Further, if such cooling means is used, the heat generation due to the polishing is eliminated. Therefore, the thermal expansion of the workpiece and the workpiece holder body is suppressed, and thus there can be produced a workpiece showing favorable flatness and good magic mirror image level. Furthermore, the same effect as described above can also be obtained when the workpiece holding surface of the workpiece holder body is coated with a resin coating film for the purpose of protection of the workpiece holding surface or co-rubbing polishing. The obtained workpiece showing high flatness, in particular when it is a semiconductor wafer, makes it possible to reduce various film thickness failures in the highly integrated device production step and improve yield of highly integrated devices.
In this case, the temperature controlling means or the cooling means may be a mechanism for flowing a temperature-controlled liquid or gas into a vacuum system way of the workpiece holder for polishing.
If such a mechanism is provided as the temperature controlling means or the cooling means, it can be easily constructed by using an existing apparatus, and both the temperature control and cooling of the workpiece holder body, the resin film and the workpiece can be performed very easily.
Further, in this case, the temperature controlling means or the cooling means may be a fluid jacket provided in the workpiece holder body.
This fluid jacket can be provided inside the workpiece holder body or around the workpiece holder body. By flowing a temperature-controlled liquid or gas into the fluid jacket, temperature control and cooling of the workpiece holder body and the workpiece can be performed very easily. Moreover, since cooling can be performed also during the polishing, the workpiece holder body and the workpiece can be maintained to be at a substantially constant temperature during the polishing. Thus, the workpiece can be finished into a workpiece showing further higher flatness.
Furthermore, in this case, the temperature controlling means or the cooling means may be a heat radiator plate provided between the workpiece holder body and the back plate constituting the workpiece holder for polishing.
If such a structure is used, the heat generated during the polishing can be discharged outside the workpiece holder for polishing via the workpiece, the resin film, the workpiece holder body and the heat radiator plate, and temperature increase of the workpiece holder body and the workpiece can be suppressed. Therefore, deformation of the holder body and thermal expansion of the workpiece can be suppressed to polish the workpiece into a workpiece showing good flatness.
Further, the material of the back plate of the workpiece holder for polishing is desirably a heat radiating material.
If a heat radiating material that shows high heat conductivity is used also for the material of the back plate of the workpiece holder for polishing as described above, the heat eliminating effect can further be enhanced as is attained by the aforementioned heat radiator plate.
The apparatus for polishing a workpiece of the present invention is a polishing apparatus comprising at least a turn table adhered with a polishing pad, means for supplying a polishing agent to a surface of the polishing pad and a workpiece holder for polishing for forcibly pressing a workpiece on the surface of the polishing pad, wherein the workpiece holder for polishing is the one described above.
By using a polishing apparatus comprising a workpiece holder for polishing in which a workpiece holder body is provided with temperature controlling means as described above, processing of workpiece showing high flatness becomes possible.
Further, the method for polishing a workpiece of the present invention is a method wherein a workpiece is polished by using the aforementioned apparatus for polishing a workpiece.
According to the polishing method performed by using a polishing apparatus comprising the workpiece holder for polishing in which a workpiece holder body is provided with temperature controlling means or cooling means as described above, a workpiece can be held with high precision, and therefore it becomes possible to perform processing of a workpiece so that it should have high flatness. Further, the polishing method of the present invention can also be applied to the so-called co-rubbing polishing, in which the workpiece holding surface is coated with a film of resin such as acrylic resin, polycarbonate resin and epoxy resin, the resin film is directly polished with a polishing pad before polishing of a workpiece so that the surface condition of the resin film should match the surface condition of the polishing pad, and then a workpiece is polished, and it can secure high flatness and good magic mirror image.
In this case, temperature of the workpiece holder body can be controlled after completion of each polishing batch by using the aforementioned temperature controlling means or cooling means.
If the temperature of the workpiece holder body is adjusted to a predetermined temperature for every batch, then the workpiece is held by vacuum adsorption, and polishing is started as described above, the heat accumulation is prevented and workpieces showing substantially no flatness fluctuation among the batches can be obtained. Thus, the yield and the productivity can be improved.
Further, in this case, when workpieces are polished in multiple batches, the polishing can be performed while controlling the temperature difference in the workpiece holder body at the start of the polishing to be within xc2x11xc2x0 C. for each batch.
Although it depends on the range of acceptable flatness, if the polishing is performed while controlling the temperature difference in the workpiece holder body, in particular, the temperature difference of inside or back side of the workpiece holder body, at the start of the polishing for every batch to be within 5xc2x0 C., preferably within xc2x11xc2x0 C., as for the recent demand for highly flat wafers, highly precise workpieces showing substantially no flatness fluctuation among the batches can be obtained.
According to the present invention, not only good flatness can be obtained in the polishing of a workpiece by suppressing the thermal deformation of the workpiece holder body and the deformation of the resin coated on the workpiece holding surface without degrading the flatness of workpieces even when the number of polishing process increases, but also workpieces showing a good magic mirror image level can be obtained.